1. Field of the Invention
This invention relates to a method and apparatus for controlling press conditions of a mechanical double action press machine, and more particularly to a method and apparatus in which a function is provided which controls various conditions which are related to press forming, such as hydraulic pressure and pneumatic pressure, automatically by a feedback method.
2. Description of the Prior Art
A deep drawing mechanical double action press machine is shown in FIG. 3. This press is equipped with upper (inner) die 38 and outer ram 39. Outer ram 39 produces a pressure aided by the distortion of outer connection rods 40 at the lower dead point of lower die 37. Hydraulic pressure is supplied to outer connection rod connector parts 7, which are located at the point where outer connection rods 40 are attached to outer ram 39, to transmit the above-mentioned pressure and absorb the overload during the forming process.
Conventionally, a control system such as shown in FIG. 4 has been utilized as a means to control the hydraulic pressure.
The outer connection rod control part 7 consists of a cylinder (a protector cylinder) 42 which is drilled in outer ram 39 and a piston 43 which is housed in cylinder 42 and to which outer connection rod 40 is attached. An oil chamber 8 is defined by the piston 43 and the cylinder 42.
A hydroblank holder 18, which has an oil chamber 18a and an air chamber 18b, is also included in a control system of the prior art. Oil chamber 18a and oil chamber 8 of the outer connection rod connector part 7 are connected to each other. A predetermined hydraulic pressure is supplied to these two oil chambers 8 and 18a from a hydraulic source 46. Hydraulic source 46 typically includes a hydraulic motor 46a, and a pressure setting valve 46b. A predetermined pneumatic pressure is also supplied to air chamber 18b of the hydroblank holder 18 from a pneumatic source 47 which typically includes a regulating valve 47a, and an accumulator 47b.
With above configuration, the hydraulic pressure is balanced with the pneumatic pressure regulated by the regulating valve 47a, i.e., the air regulator pressure. Hydraulic pressure is applied to oil chamber 8 of outer connection rod connector part 7, and thus determines the pressure of the outer ram 39. In this case, when the predetermined pressure of the regulating valve 47a is varied, the hydraulic pressure balancing with it also changed as a result, hence varying the pressure of the outer ram 39.
When the internal pressure of an outer balance cylinder (the reference numeral 48 of FIG. 3) connected to the upper part of the outer ram 39 is regulated by a regulator (not shown) to a magnitude just sufficient to push up both the weight of the upper (inner) die and the weight of the outer ram at the time of the lower dead point of the press, an outer (outward) load corresponding to above predetermined hydraulic pressure is transmitted. Conversely, if the pressure is regulated by the regulator (not shown) to a magnitude exceeding the value just enough to push up the weight of the upper (inner) die (hereinafter referred to as inner die) and the weight of the outer ram at the time of the lower dead point of the press, the outer load with the magnitude obtained by reducing the excess upward pressure of the outer balance cylinder 48 (excess value of the regulator pressure) from the pressure corresponding to above predetermined hydraulic pressure is transmitted.
Also if the hydraulic pressure in the oil chamber (which is predetermined at the time of the upper dead point of the outer) is changed at the time of the lower dead point, the compression property of the operating oil is changed so that the above outer load is changed.
The prior art mentioned above has thus posed a problem as described below. The internal pressure of air chamber 18b of hydroblank holder 18, the internal pressure of outer balance cylinder 48 and the internal pressure of oil chamber 8 of the outer connection rod connector part 7 at the upper dead point of the press, greatly affect the forming characteristics of the pressing in the prior art. However, these pressures are manually set in advance by an operator at the upper dead point of the press, prior to the forming. Thus, the change in the desired pressure values at different times during the forming process cannot be determined.
Therefore, in order to manually preset the pressure in the prior art, not only have much labor and time been required, but also an abnormal change of the preset pressure at the time of the upper dead point cannot be determined, and hence forming has been carried out even when the outer load has shown an abnormal value at the time of the lower dead point. Thus, it has been difficult to maintain stability during the process.
Moreover, abnormal situations such as oil leakage in oil chambers 8 and 18a, oil accumulation in air chamber 18b and malfunction of hydroblank holder 18 at the time of forming have been very difficult to detect. Therefore, when such factors have caused a press to malfunction, those factors could not be identified, and the malfunction was inadequately corrected, for instance by adjusting the dies.